1. Field of the Invention
The present invention relates to a switching device that controls a large amount of current, the switching device including a power-switching element such as a switching transistor, and more particularly to such a switching device used for controlling operation of a motor-assisted steering device.
2. Description of Related Art
An example of a conventional power-switching device is shown in FIG. 7. A switching element 131, such as a power-MOS-FET or an IGBT, is mounted on a heat-sink block 181 having a flat mounting surface 182. A connecting lead 131b led out from the switching element 131 is electrically connected to a printed circuit board 161 by soldering. Heat generated in the switching element 131 is dissipated through the heat-sink block 181. Another example of this kind of power-switching device is disclosed in JP-A-2000-203437. This power-switching device is used in a circuit for controlling a motor-assisted steering device for an automotive vehicle. A semiconductor switching element is mounted on a surface of a metallic substrate connected to a heat-sink.
In the conventional switching device shown in FIG. 7, a large amount of current, e.g., 40–80 amperes, flows through the connecting lead 131b. The switching element 131 is mounted on the flat mounting surface 182 that is parallel to the printed circuit board 161, and a space for clearing a thickness of the switching element 131 and a height of a mounting screw has to be provided between the flat mounting surface 182 and the printed circuit board 161. Therefore, the connecting lead 131b has to be long enough to cover the space. Accordingly, heat is generated in the connecting lead 131b by a large amount of current flowing therethrough. Especially, the heat is generated in a portion of the connecting lead 131b having a small cross-sectional area.
In the power-switching device disclosed in JP-A2000-203437, a power-switching element is mounted on a metallic substrate and other electronic components are mounted on another circuit substrate. Therefore, two substrates are required and both substrates have to be electrically connected to each other. Accordingly, the structure of the power-switching device becomes complex and its manufacturing cost becomes high. Further, though the metallic substrate has a high heat-dissipation ability, only a single layer circuit can be printed on the metallic substrate, and therefore it is unavoidable that a circuit impedance becomes high, accompanied by generation of higher noises.